As electronic devices become more powerful and more compact, requirements have grown for compact, high-performance transformers, choke coils, filters and the like. Moreover, there is a trend toward the use of larger electrical currents. Up to now, ferrite has been the most commonly used low-cost magnetic material. However, a disadvantage of ferrite is that it has a low saturation magnetic flux density, and is thus unable to cope with ever larger electric currents.
By contrast, soft magnetic metal powder has a higher saturation magnetic flux density than ferrite and can thus accommodate large currents. However, it has not only low electrical resistance and large eddy current loss but also poor resistance to oxidation because iron is its main component. In order to overcome these disadvantages, a composite-type magnetic core has been developed, as disclosed, for example, in Japanese Laid-Open Patent Publication No. 2003-318014.
In order to give the above-described composite-type magnetic core itself high oxidation-resistant properties, a coating method is employed in which the magnetic core is coated with a resin such as an epoxy resin or a fluorocarbon resin. However, if the thickness of the resin coating is too thin, the magnetic core will have insufficient oxidation resistance. On the other hand, increasing the thickness of the coating layer increases the size of the magnetic core, which makes it difficult to satisfy the demand for greater compactness. Where a combination of E-type magnetic cores are used, covering the surfaces of the E-type magnetic cores where they join with organic material widens the magnetic gap, which reduces inductance and also leads to unevenness in inductance caused by inconsistencies in the thickness of the organic material coating.
In order to eliminate such deficiencies there exists a method involving masking the contact surfaces and coating the remaining surfaces with resin. However, such a method causes a decrease in the oxidation resistance of the contact surfaces.